Several processes have been disclosed for the liquefaction of cryogens such as helium.
In U.S. Pat. No. 3,828,564, a process for liquefaction of a cryogen such as helium is disclosed. This process comprises cooling and liquefying said gas by indirect heat exchange with a separate refrigerant circulating in a closed refrigeration cycle. The heat exchange is conducted with the refrigerant from a single refrigerant cycle, said refrigerant being subjected to both engine expansion and at least partially isenthalpic expansion, whereby the refrigerant is cooled sufficiently to effect liquefaction of all the cryogen in a single pass, thereby avoiding the necessity of additional compressor or purification capacity for recycled gas.
In U.S. Pat. No. 3,932,158, an object is cooled by a collant operating with a single or multi-stage coolant cycle in which the coolant, in the last stage, is partially expanded, cooled in a separator-evaporator and fed to the object to be cooled. At least a portion of the coolant fluid, following passage through the object, is expanded through a throttle to form a liquid-gas phase mixture which is separated in the separator-evaporator, the gas phase being recirculated. The expansion of the coolant fluid prior to entry into contact with the object is carried out according to the patent in one or more ejectors whose suction side or sides draws a portion of the cooling fluid from part of the cycle elsewhere into the stream fed to the object to increase the mass flow.
In U.S. Pat. No. 4,169,361, refrigeration is produced by compressing a refrigerant and expanding the refrigerant isentropically in a nozzle. At least a part of the expanded refrigerant is passed in indirect heat exchanging relationship with the portion of the refrigerant prior to expansion. An expansion engine can be used to work-expand a portion of the compressed refrigerant with the expanded gas returned to the compressor. The balance of the compressed stream is expanded in the nozzle.
In U.S. Pat. No. 4,267,701, a helium liquefaction plant is disclosed, wherein a compressor includes first, second and third stages and a precooling section includes first, second and third turboexpanders in series between high and low pressure lines of a heat exchanger. A portion of the medium pressure gas at the output of the second turboexpander is directed back through the heat exchanger and mixed with the output of the first compressor stage. The third turboexpander is positioned between the medium and low pressure lines.
In U.S. Pat. No. 4,498,313, a helium gas-refrigerating and liquefying apparatus is disclosed, which comprises: a neon gas-refrigerating and liquefying circuit which precools helium gas and comprises a turbo type compressor, heat exchangers, turbo type expansion machines and a Joule-Thomson valve and a helium gas-refrigerating and liquefying circuit which comprises a turbo type compressor, heat exchangers, an expansion turbine and a Joule-Thomson valve, the former circuit system being constructed to associate with the latter circuit system so as to further cool the precooled helium gas in the latter circuit system by heat exchange therewith.
None of the aforementioned processes disclose how to handle the problem of recycling warm vapors to the main liquefier, which are generated by the process and during the loading of product. Two solutions to this problem have been known and used in commercial practice. One method was to eliminate the generation of warm vapors and the other method was to reliquefy the warm vapors.
The first method, tried with only partial success, was to circulate helium, cooled by liquid nitrogen, through product trailers. Unfortunately, many of these trailers are effectively partitioned lengthwise by several transverse anti-slosh baffles. In some instances the vapor vent line of the trailer is in the front of the inner tank, some in the middle, and some in the rear. In the latter case, the circulating helium effectively by-passed most of the inner tank, and the tank could never cool to circulate temperature.
The second method and present standard practice is the installation of a reliquefier. The warm helium vapors are returned to a reliquefier unit which contains a series of heat exchangers and compression and expansion equipment. About 80-90%, of the warm vapors are reliquefied and returned to the storage tank; the balance of the warm helium vapor is transferred at ambient conditions to the main liquefier unit.
The reliquefier can also be used to make the liquefier independent of the tank by operating the reliquefier to process tank vapors, in addition to the warm vapors generated by heat leak, which would normally be sent back to the liquefier.
Despite the advances made in the art, the art as represented above has failed to disclose an efficient method for recycle of warm vapors back to the main liquefier.